This patent specification relates to the field of gas turbine engines, and more particularly to a steam injection nozzle system for the combustion liner of a gas turbine engine for enhancing the power output and efficiency of the gas turbine engine.
A gas turbine engine is a heat engine that is operated by a gas rather than being operated, for instance, by steam or water. The two major application areas of gas turbine engines are aircraft propulsion and electric power generation. A detailed description of gas turbines is provided in William W. Bathie, Fundamentals of Gas Turbines (John Wiley and Sons, Inc. 1996), which is hereby incorporated by reference.
The burner section of a gas turbine includes a combustion chamber which is designed to burn a mixture of fuel and air and to deliver the resulting gases to the turbine at a temperature not exceeding the allowable limit at the turbine inlet. The burners, within a very limited space, must add sufficient heat energy to the gases passing through the engine to accelerate their mass enough to produce the desired thrust for the engine and power for the turbine.
Combustion chambers are lined with combustion liners. FIG. 1 illustrates a typical combustion liner. The holes supply primary, secondary, and cooling air for the gas turbine operations. As illustrated in FIG. 1, the fuel comes from one end of the combustion liner (in this case, from the left of the page) supplemented by a pre-mixed airflow. A plurality of jets, indicated by holes along the combustion liner, supply turbulence and a secondary air supply for the combustion regions. These jets are depicted by the holes of Sections 1, 2, and 3, which operate to satisfy various air requirements for achieving a quality combustion system.
Section 4 is usually a much larger area, with the air being supplied by higher-level jets to create turbulent mixing. In this process, the combustion flame products are mixed with the compressor air to reach a final homogeneous working fluid at designated operating turbine inlet temperatures (TIT). There are also many small cooling holes along the wall of the liner to keep the liner metal temperature down. The final flow of the combustion liner exhaust is directed into a transition piece which connects the air flow cross section of a typical combustion can (typically a cylinder-shaped burner) to a turbine nozzle bank segment. This design is typically used with all or nearly all modern, ground-based gas turbines.
Heavy duty gas turbines with long combustion liner designs frequently use reverse flow combustion liners to accommodate a long flame. Reverse flow combustion liners are used in other combustion liner variations as well. In general, to compensate for the length of the combustion liner, compressor discharge air ordinarily flows backwards into the envelope or combustion wrapper of the combustion cans, then reverses its direction, moving through the combustion liner and reaching a designated turbine inlet temperature at the first row of nozzles in the gas turbine. The gas turbine""s performance is related to the turbine inlet temperature, and the first stage nozzle and blades have maximum metal temperature limitations. These temperature limitations are determined by the materials used. Even though today""s turbines use single crystal metals, they must still employ various cooling means to keep the metal temperature down.
The combustion liner design has been carefully engineered over the years so that the flame can operate with a high ratio of turndowns to provide an efficient startup and to provide low load operating conditions. Using a diffusion flame, the fuel is mixed with an oxidizer before combustion. The oxidizer then diffuses to the flame envelope, allowing the oxidizer and the fuel to reach a stoicheometric ratio where the flame resides. A diffusion flame is the preferred method for increasing the diffusion rate of oxidants to the flame envelope. Because the flame always resides at the stoicheometric ratio surface, the gas turbine inlet temperature is controlled by running dilution air downstream along the length of the flame to reach an appropriate mixture and a designated homogenous design temperature for the gas turbine. The dilution holes are strategically located to provide an air jet which creates internal turbulence. Because this turbulence causes the pressure to drop, it also reduces the fluid working potential. The air jets must therefore attempt to provide turbulence at a level that will result in a minimal pressure drop across the combustion liner to avoid loss of the fluid working potential.
One area of gas turbine engines which needs improvement is the area of power output and efficiency. The Advanced Cheng Cycle, conceived by the Applicant of the present disclosure, is a massive steam-injected gas turbine that uses steam to augment its power output. Steam is injected into the gas turbine ahead of the first turbine nozzle bank just downstream of the combustion region. Steam injection has previously been employed as a power boost on some gas turbine engines. The injection, however, has been traditionally limited to about 5 to 9% of the air flow in order to avoid causing compressor stall and flame instability.
In previous systems, steam and air has been injected before the combustion liner; that is, steam was not injected directly into the combustion liner. The steam injection point has previously been the compressor exit plenum area. By this method, steam enters the combustor through all combustor liner admission areas: the primary zone, the dilution zone, and the cooling louvers. The combustor pressure drop increases with increased steam flow, depending on the steam-air ratio. Therefore, although the effect on combustion efficiency could be minimal, the additional mass going through the holes on the combustion can requires a higher pressure drop. Furthermore, steam carried in the air modifies the combustion of air by reducing the relative concentrations of both oxygen and nitrogen. Dilution of the oxygen in the composition lowers the combustion reaction rate.
Moreover, in recent years, air pollution has emerged as a major concern in the field of chemical engineering, and reducing air pollution is a secondary goal of the disclosure herein. The predominant emissions from gas turbines are the oxides of nitrogen, or NOx, which are one of today""s leading components of air pollution. The most prevalent NOx emissions are nitric oxide, NO, and nitrogen dioxide, NO2. The diffusion flame temperature emanating from gas turbines produces these NOx emissions. If the flame front receives insufficient oxygen or turbulence, the resulting concentration of carbon monoxide can become an additional factor in highly polluted air.
In light of the air pollution problem, gas turbines now use a dry, low NOx, combustion liner. This type of combustion liner maintains both a lower pressure level and a higher turbulence level than ordinary combustion liners in order to achieve the low flame temperature necessary to reduce NOx emissions.
The Advanced Cheng Cycle demonstrates that NOx emissions have decreased substantially from previous simple cycle combustion liner designs. However, the reverse flow combustion liner differs somewhat from the aeroderivative gas turbine design by allowing steam to easily mix with the compressor discharge before it enters the combustion liner.
Some of the current gas turbine manufacturers have injected steam concentric to the fuel nozzle as a means of lowering the NOx emissions. The optimum steam injection using that particular method reaches a limit of flame stability at a steam-to-fuel ratio of approximately 1:1. However, a problem with this amount of steam is injection into a combustion liner is that power augmentation suffers.
A secondary method of steam injection involves injecting the steam at the plenum chamber. Here, the steam will mix with the compressor air and the mixture will flow around the combustion liner, entering the combustion liner through the various dilution and cooling holes. However, there is a problem in that when the steam-to-air ratio reaches about 9%, the flame becomes unstable.
The system and method disclosed herein overcome the above-described difficulties.
An object of this patent specification is to increase the power output and efficiency of gas turbine engines. A secondary goal is to reduce the level of NOx emissions emanating therefrom. Another object is to provide a system in which steam is injected in various amounts into existing dilution holes of the original combustion liner design without disturbing the air flow distribution thereof. The disclosure herein teaches to use steam pressure and its produced momentum to eject air through the original dilution holes. This both minimizes the pressure drop required to create turbulence and provides turbulence itself, using the momentum of the steam jet inside the combustion liner. Along with the increased turbulence inside the combustion liner, the flame temperature is reduced by adding the steam, which has a very high heat absorption capacity, to the existing mixture of oxygen and nitrogen (the major components of air). Because the steam enables the mixture to absorb the heat of combustion, the flame temperature is reduced. The turbulent mixing will also provide a sufficient supply of oxygen to potentially reduce the amount of carbon monoxide.
Injection of steam directly into the combustor liner through the dilution air holes as taught by the disclosure herein has the desirable feature that little or no steam is admitted directly to the primary combustion zone, the area very close to the fuel injection nozzles. This has the result that the reaction kinetics inside the combustion liner are unaffected by the steam injection.
Thus, an objective of the disclosure herein is to inject steam through a plurality of dilution holes, proportional to the total airflow, without altering the air distribution already designated for those dilution holes. Another objective is to reduce the pressure drop required to inject steam across a combustion chamber at the onset of the steam injection rate. A further objective is to increase the turbulence diffusion rate for combustion through the process of turbulent mixing, which sufficiently reduces both the flame temperature and the NOx emissions.
A further objective is to provide a consistent mixture of the main combustion products, the dilution cooling air, and the steam to create a steady or uniform temperature profile before the mixture enters the first nozzle of the gas turbine. By establishing a consistent temperature profile, the mixture can reduce the metal temperature of the hot streaks and hot spots in the first stage nozzle, prolonging its life and increasing its accurate function.
The disclosure herein therefore describes efficient methods of injecting steam into gas turbine combustion liners. The disclosure targets all combustion liner variations, and particularly relates to reverse flow combustion liners.
In summary, the disclosure herein in one embodiment provides a steam injection nozzle system for injecting steam into a combustion liner of a gas turbine for enhancing power output and efficiency of the gas turbine, the combustion liner having a plurality of dilution holes for supplying air to a combustion chamber to create turbulence, the steam injection nozzle system comprising: a steam manifold surrounding the combustion liner and having a plurality of steam injection nozzles, disposed opposite corresponding dilution holes, for injecting the steam directly into the combustion liner through the dilution holes, wherein the steam is injected without substantial alteration of an existing amount of air through the plurality of dilution holes, resulting in a constant amount of air through the plurality of dilution holes independent of the amount of steam injected therein within a predetermined range of steam.
The predetermined range of steam injected may be between approximately 9% and approximately 35% by weight, inclusive, of the air flowing through the combustion liner. Alternatively, the predetermined range of steam injected may be less than or equal to approximately 35% by weight of the air flowing through the combustion liner. The steam injection nozzle system is preferably designed such that each steam injection nozzle disposed opposite a corresponding dilution hole is not in contact with the combustion liner.
The relationship between the steam nozzle diameter design and the size of the dilution holes of the combustion liners will be discussed in detail below, along with the effect of this relationship on combustion and emissions of gas turbines.